This invention relates to an improvement of a die cast machine, more particularly an improved fluid pressure operating circuit for the injection cylinder described in U.S. Pat. No. 3,891,126 dated June 24, 1975.
According to this patent, the energy of the compressed gas contained in an accumulator is used for moving the piston of the injection cylinder of a die cast machine at an extremely high speed without accompanying an objectionable water hammering phenomenon thereby decreasing the time required for raising the pressure of cast molten metal and eliminating the problems of dimensional inaccuracy and fins caused by the water hammering phenomenon.
However, as a result of further investigation it was found that there remains problems to be solved as follows.
More particularly, in most (more than 90%) of the metal moulds now being used commercially a high rate of rise in the pressure applied to the molten metal at the final stage of injection is desirable but several % of the metal moulds have gaps between the mating surfaces of the mould halves due to a wear, deformation and manufacturing error of the metal moulds. When such metal mould is used if the rate of rise in the pressure applied to the molten metal were too high fins or flashes would be formed. Accordingly, it is necessary to slightly slow down (of the order of 1/1000 second) the rate of pressure rise at the sacrifice of the quality of the casting.
It has been considered that it is desirable to inject the molten metal at a constant speed into the mould cavity by the injection plunger and the die cast machine has been designed to meet this requirement. Thus, in the first stage of the injection, for the purpose of preventing a wave from being formed on the surface of the molten metal poured into an injection sleeve and exhausting the air in the injection sleeve to the outside of the metal mould through an air vent, it is usual to move the injection plunger at a relatively slow constant speed or to gradually accelerate the plunger from a relatively low initial speed so that the plunger will attain a constant high speed when the molten metal reaches the gate of the metal mould.
However, the temperature and fluidity of the molten metal decrease with time so that the resistance to the movement of the injection plunger increases with the injection stroke. Accordingly, the injection plunger of the prior art machine does not move at a constant speed during the injection stroke but decelerates as the stroke proceeds. With such injection speed characteristic, as the speed of the molten metal flowing through the gate of the metal mould during the later stage becomes smaller than that of the molten metal flowing during the early stage with the result that the molten metal injected into the mould cavity becomes discontinuous thus entraining air bubbles in the casting, dislocations (phenomenon wherein separated metal portions do not fuse again), and surface defects of the cast products.